Hi Jenesaisquoi, I have attended the Tattoo several times: very impressive show and yes, certainly recommend it. The shows usually sell out or close to selling out so I would not suggest waiting to buy tickets 'till the day you come into port. http://www.nstattoo.ca/
If I recall correctly, the show is approx. 3 hours so that would have it finishing at 5:30pm Atlantic time and it looks like the Glory leaves at 7pm so timing is very good.
Btw, the Metro Centre is approx. 20 min. walk from the pier. So before the show, you could do a walk along the water front first, maybe stop in the Maritime Museum or visit Citadel Hill fort.
Your 8 yr old may well enjoy the show, there are the military bands and also competitions that are always exciting.
To be honest, I would be surprised if the ship would ever skip port due to fog. You may want to call/email Carnival to ask for info. on what percentage of times Halifax had to be skipped. My guess is that the ship has never missed Halifax.
Steve (Halifax, Nova Scotia)